The present invention relates generally to optics, and more particularly, to a an adaptive optical amplifier for wavelength division multiplexing WDM systems.
The following prior documents referenced in the application are discussed, not material to patentability of the claimed invention, nevertheless provide additional information.    [Chraplyvy] A. R. Chraplyvy, J. A. Nagel, and R. W. Tkach, “Optical transmission system equalizer”, U.S. Pat. No. 5,225,922, 1993.    [Li] J. Li, S. Yuen, “WDM system equalization with EDFA optical amplifiers”, U.S. Pat. No. 6,323,994 B1.    [Kinoshita] S. Kinoshita, “Optical communication system and optical amplifier”, U.S. Pat. No. 6,452,719 B2, 2002.    [Ji] P. N. Ji, “Software defined optical network”, Proceedings of 1 lth International Conference on Optical Communications and Networks (ICOCN 2012), paper THU-07, November, 2012, Thailand.
Optical amplification is a key element in an optical communication network. It amplifies the power level of the optical signal to compensate for the attenuation in fiber transmission and/or the insertion loss caused by optical components. In wavelength-division multiplexing (WDM) optical systems, multiple optical channels with different wavelengths/frequencies are transmitted concurrently over a single fiber. Optical amplifiers in WDM systems are able to amplify multiple WDM channels simultaneously to save hardware cost. They are also called repeaters. Depending on the location and usage of optical amplifier, there are different amplifier types in the WDM network, including boost amplifiers that amplify the signal exiting the transmitter, in-line amplifiers (or called line amplifiers) that amplify the signal along the transmission path, and pre-amplifiers that are placed before the receiver to amplify the signal to the suitable level to be detected. In terms of the technologies, Erbium-doped fiber amplifier (EDFA) is the most common type of optical amplifier in WDM systems. Other types include Raman amplifier (RA) and semiconductor optical amplifier (SOA). This invention applies to any amplifier type and any amplification technology.
For these optical amplifiers for WDM optical communication systems, the distribution of power among different WDM channels at the amplifier output are usually not uniform (such as having a tilt or ripple), due to the physical characteristics such as the gain media properties, etc. The non-uniformity causes different gains among different channels, and in turn causes different optical signal-to-noise-ratio (OSNR) figures. Those channels with a relatively low OSNR and low received power could result in an excessively high bit error rate. Therefore there is a need to balance/equalize the amplifier power.
Furthermore, as the WDM network becomes more dynamic and flexible, the conventional single line rate, uniform modulation format system is no longer guaranteed. Instead, different WDM channels can have different line rates, use different types of modulation format, occupy different amount of spectral width, etc. They also have different transmission distance and requirements. Therefore the required OSNR level and the optimum power level for each channel might be different in such heterogeneous WDM network, and thus the amplification requirements are also different.
A straightforward method is to adjust the input power level based on the feedback from the amplifier output [Chraplyvy], however this is not practical because this feedback technique typically requires numerous iterations, therefore is slow to complete. This is especially true as the number of channels increase. Also, this technique requires that the transmission power for each individual WDM channels can be adjusted over a large dynamic range. As the dynamic range increases, the complexity and cost of the transmission power adjusters required within the multiplexer also increase.
Another method to compensate for the tilt at the amplifier output is to set up a variable attenuator between the 2 stages of a single amplifier [FIG. 3 of Li]. It is common for an in-line amplifier to have 2 stages. By applying a suitable amount of attenuation between the 2 stages, the gain tilt slope (difference in the output power between low frequency end and high frequency end) can be mitigated. However this technique only handles the general tilt across the entire spectrum and cannot balance among individual channels.
Another method is to change the attenuator into an array of attenuators, one for each optical channel or band. Such attenuator array can be placed between the 2 amplifier stages [FIG. 7 of Li], or at the output of the amplifier to attenuate the channels/bands that has higher power (a common practice). A wavelength blocker (WB), which is an integrated optical device that consists of spectrum disperse element, array of switches/attenuators, and spectrum combination element, is commonly used. Even though this method can produce more equal power levels at the output, the optical power utilization is not efficient. The power from the high power channels is discarded (wasted) through the attenuation process, and not transferred to the other channels. The OSNR of lower power channels is not improved. Also, such equalization is only based on the power level at the amplifier output, and does not take into consideration the signal type and characteristics of individual WDM channels, therefore it is not suitable for mixed line rate system.
Another method is to use a supervisory circuit to detect the number of active WDM channels in the fiber through spectrum monitoring, and then control the amplifier based on this information [Kinoshita]. The advantage of this method is that it uses actual signal information to balance the amplifier, however the only information it can consider is the number of channels, and not other information such as the modulation format, data rate, etc.
Accordingly, there is a need for a latching WSS/WB the can be used in reconfigurable BU in submarine network, that overcomes the shortcomings of prior efforts.